These tumor-initiating actions linked to RB loss occur both in stem cells, where normofunctional RB keeps them quiescent, their usual state, and in mitotically inactivated differentiated cells, in which RB mutation allows these post-mitotic cells to reintegrate into the progenitor compartment, and especially in the proliferative pool of cells, which are called amplifying transitory cells in the oral epithelium, which constitute an intermediate step between stem cells and differentiated post-mitotic cells. Here, RB1 is linked to neoplasm.